1. Field of the Invention
The present invention relates to radar detection circuits and more particularly to ultra-wideband (UWB) RF pulse detection circuits. The invention can be used to detect pulses for UWB radar rangefinders, motion sensors, time domain reflectometers (TDR), pulsed laser sensors, radiolocation systems, and communication systems.
2. Description of Related Art
In 1918, Edwin H. Armstrong invented the superheterodyne radio which translated a high incoming radio frequency (RF) to an intermediate frequency (IF) using a local oscillator and a mixer. Shortly thereafter, the mixer and oscillator were combined into a single function using a single vacuum tube which functioned as both a local oscillator and a mixer. A device that functions as both an oscillator and a mixer can be termed a self-oscillating mixer (SOM). The oscillations in early radio SOMs were continuous wave (CW) and were not phase coherent with the incoming RF, i.e., there were no RF cycles and oscillation cycles that repeatedly aligned on each cycle or on every Nth cycle, where N is an integer. Later versions of the SOM employed transistors. The SOMs in superheterodyne receivers produce an IF output that is amplified and then detected—the detection function is not part of the SOM.
The superregenerative receiver, patented in 1922 by Armstrong, employs a pulsed SOM that serves as a pulsed oscillator and pulsed mixer. The SOM is pulsed repeatedly by drive, or quench, pulses at a moderate rate, typically just above the audio frequency band. Very small levels of incoming RF influence the start-up phase and duration of the pulsed oscillations in the SOM, which in turn varies, on an amplified scale, the average DC current through the SOM. The output of the SOM is coupled to a lowpass filter (LPF), which acts as an integrator that integrates multiple pulses from the SOM to produce a detected output signal. The oscillations in the SOM are not phase coherent with the drive pulses; rather, the oscillations are of random phase when no incoming RF is present and are phase-locked to the incoming RF when the RF is strong. Due to phase-locking, a beat frequency cannot be produced; rather a phase-less current pulse is produced by the pulsed SOM, i.e., the response is a magnitude response with no phase information. Superregenerative receivers are commonly used in receivers for garage-door and automobile doors. An example of a Micropower superregenerative receiver is disclosed in U.S. Pat. No. 5,630,216, “Micropower RF Transponder with Superregenerative Receiver and RF Receiver with Sampling Mixer,” by the present inventor, Thomas E. McEwan. A recent example of a microwave superregenerative receiver is given by Moncunill-Geniz et al in an IEEE paper entitled “An 11-Mb/s 2.1 mW Synchronous Superregenerative Receiver at 2.4 GHz.” This paper discloses a method of taking one sample per data bit of received RF as opposed to taking several samples or more per bit in the classical superregenerative receiver. As in prior super-regenerative receivers, the oscillations are allowed to build up slowly to maximize their possibility of becoming phase-locked to the incoming RF and not to the pulse driver.
A very recent example of a SOM is disclosed by Winkler et al in an IEEE paper entitled “Integrated Receiver Based on a High-Order Subharmonic Self-Oscillating Mixer.” CW operation is employed with the RF input being at a harmonic of the oscillation frequency. One aspect of the SOM disclosed is an arrangement to cancel local oscillator leakage out the RF input port using two SOMs and a 180° combiner.
Another relevant technology is the ultra-wideband (UWB) sampler. Early examples of UWB samplers can be found in sampling oscilloscopes, such as the Tektronix model S-2 sampling head, dating to the 1960's. The S-2 bandwidth extended from DC to 6 GHz. It operated by short-term integrating an incoming signal during its sampling aperture, and then capturing the integrated sample for longer storage with a sample-hold (S/H) circuit. A long term integrator could be selected to smooth the samples and lower displayed noise. A UWB impulse radar system based on such a sampler was published by Bennett and Ross in 1978. Since each impulse is individually sampled and held, individual, pulse-by pulse receiver processing is employed.
In 1993, two patents disclosed a new type of impulse radar based on integrated-pulse receiver processing. These were U.S. Pat. No. 5,345,471, “Ultra-WideBand receiver” and U.S. Pat. No. 5,361,070 “Ultra-Wideband Radar Motion Sensor,” both by the present inventor. The '471 receiver samples multiple pulse repetitions and coherently integrates them in one step, using a gated integration capacitor connected directly to the antenna. The integrated output from the capacitor is a non-pulsed, continuous, detected output signal that is easy to process using simple low-bandwidth circuitry. This new technology is dubbed Micropower Impulse Radar (MIR) and is in wide commercial use through patent licensing.
One potential limitation to impulse type samplers is their bandwidth can be too large for some applications, e.g., their bandwidth extends to zero, even though antennas cannot transmit and receive zero Hertz or relatively low frequencies (relative to their size). This excessive bandwidth can, for example, substantially degrade S/N when operating in the 24-31 GHz UWB band while using a sampler that spans from DC to 31 GHz. It is clearly preferable to use a matched bandwidth sampler. For example, UWB radars can be made to operate in the 5.4-7.1 GHz region under FCC Part 15.209 regulations, which cover general emission limits. This band is not formally allocated for UWB operation, but UWB operation is permitted provided the emission level is extremely low (about one nanowatt average transmit level). The most efficient receiver for this application—where every decibel counts with nanowatt transmitters—is one with a bandwidth matched to the transmitter bandwidth, and not the classic UWB impulse sampler. Prior approaches to matched bandwidth employ conventional filters and mixers, which are inefficient, bulky, costly, and require power-hungry preamplifiers. Presently, there is no approach that provides a compact, low cost, low power, matched bandwidth, sampling solution.